Name: Optically Trapping Large Metallic Particles in Air Using a Boat Trap with Direct-Drawn Sidewalls (Video) | JoVE Journal
Uploaded: 2026-03-27T13:10:00.000+00:00
Duration: 10 min 36 s
Description: Learn to build a reconfigurable boat-shaped optical trap for capturing and manipulating large metallic particles in air using scanned laser beams. This method enables stable, long-duration trapping for plasmonic studies.

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This work was supported by the National Science Foundation under Grant No. 2234534.

<ol><li>Lin, J., Li, Y. Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Optical+trapping+and+rotation+of+airborne+absorbing+particles+with+a+single+focused+laser+beam%5BTitle%5D)+AND+%22Appl+Phys+Lett%22%5BJournal%5D)">Optical trapping and rotation of airborne absorbing particles with a single focused laser beam.</a> Appl Phys Lett. 104 (10), 101909(2014).</li>
<li>Smalley, D. E., Bove, V. M., Barabas, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photophoretic-trap+volumetric+display%5BTitle%5D)+AND+%22Nature%22%5BJournal%5D)">Photophoretic-trap volumetric display.</a> Nature. 553 (7689), 486-490 (2018).</li>
<li>Shvedov, V. G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Selective+trapping+of+multiple+particles+by+volume+speckle+field%5BTitle%5D)+AND+%22Opt+Express%22%5BJournal%5D)">Selective trapping of multiple particles by volume speckle field.</a> Opt Express. 18 (3), 3137-3142 (2010).</li>
<li>Sil, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dual-mode+optical+fiber-based+tweezers+for+robust+trapping+and+manipulation+of+absorbing+particles+in+air%5BTitle%5D)+AND+%22J+Opt%22%5BJournal%5D)">Dual-mode optical fiber-based tweezers for robust trapping and manipulation of absorbing particles in air.</a> J Opt. 19 (12), 12LT02(2017).</li>
<li>Sil, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Ultrastable+three-dimensional+photophoretic+trap+in+air+facilitated+by+a+single+multimode+fiber%5BTitle%5D)+AND+%22ACS+Photonics%22%5BJournal%5D)">Ultrastable three-dimensional photophoretic trap in air facilitated by a single multimode fiber.</a> ACS Photonics. 11 (1), 159-170 (2024).</li>
<li>Shvedov, V. G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Giant+optical+manipulation%5BTitle%5D)+AND+%22Phys+Rev+Lett%22%5BJournal%5D)">Giant optical manipulation.</a> Phys Rev Lett. 105 (11), 118103(2010).</li>
<li>Shvedov, V. G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Robust+trapping+and+manipulation+of+airborne+particles+with+a+bottle+beam%5BTitle%5D)+AND+%22Opt+Express%22%5BJournal%5D)">Robust trapping and manipulation of airborne particles with a bottle beam.</a> Opt Express. 19 (18), 17350-17356 (2011).</li>
<li>Wang, C., Gong, Z., Pan, Y. -L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Optical+configurations+for+photophoretic+trap+of+single+particles+in+air%5BTitle%5D)+AND+%22Rev+Sci+Instrum%22%5BJournal%5D)">Optical configurations for photophoretic trap of single particles in air.</a> Rev Sci Instrum. 87 (10), 103104(2016).</li>
<li>Svoboda, K., Block, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Optical+trapping+of+metallic+Rayleigh+particles%5BTitle%5D)+AND+%22Opt+Lett%22%5BJournal%5D)">Optical trapping of metallic Rayleigh particles.</a> Opt Lett. 19 (13), 930-932 (1994).</li>
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<li>Sasaki, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Optical+trapping+of+a+metal+particle+and+a+water+droplet+by+a+scanning+laser+beam%5BTitle%5D)+AND+%22Appl+Phys+Lett%22%5BJournal%5D)">Optical trapping of a metal particle and a water droplet by a scanning laser beam.</a> Appl Phys Lett. 60 (7), 807-809 (1992).</li>
<li>O’Neil, A. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Intrinsic+and+extrinsic+nature+of+the+orbital+angular+momentum+of+a+light+beam%5BTitle%5D)+AND+%22Phys+Rev+Lett%22%5BJournal%5D)">Intrinsic and extrinsic nature of the orbital angular momentum of a light beam.</a> Phys Rev Lett. 85 (5), 764-767 (2000).</li>
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<li>Mirzaei-Ghormish, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Optical+trapping+of+large+metallic+particles+in+air%5BTitle%5D)+AND+%22Phys+Rev+Appl%22%5BJournal%5D)">Optical trapping of large metallic particles in air.</a> Phys Rev Appl. 22 (6), 064042(2024).</li>
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The authors declare that they have no competing financial interests.

The successful implementation of this trapping technique relies on precise modulation of the AOM frequencies and the correct alignment of optical components. One of the most critical steps is ensuring that the scanning laser beam generates a well-formed parabolic potential.&#160; While shorter focal lengths and higher numerical apertures generally improve trap stiffness and confinement, the convenience of a long working distance objective for particle loading and observation outweighed these benefits in this application....

This work presents a method for the precise capture, placement, and manipulation of large metallic particles in air using a dynamically generated boat trap. Prior approaches—such as those employing loosely focused Gaussian beams1, aberration traps2, speckle fields3,4,5, or vortex beams6,7—suffer from key limitations, including rotational motion1, multiple stable trapping loci2,3,4,5, or a lack of a single, well-defined trapping site6,7, which, in some geometries, may lead to hopping between multiple stable trapping loci3,7 (see Figure 1A).  One noteworthy exception is the upward-directed axicon trap, which provides a single trapping locus, supports top-loading, and—at least theoretically—permits lateral translation8. The method described here retains those advantages while introducing three critical improvements: (1) robust trapping of metallic particles, (2) a horizontal beam geometry, and (3) real-time reconfigurability of the trapping potential via direct-drawn optical walls. 
Approaches for trapping gold particles to date have typically been performed in water9,10,11, may have trap characteristics that impart rotational motion12, or employ counter-propagating beams that inhibit the flexible translation of a trapped particle13. To overcome these challenges for solid gold particles with diameters greater than 1 μm, a dynamically modulated photophoretic 'boat' trap is employed.  
This method is applicable to a wide range of particle sizes and types, from 20 nm carbon nanosphere clusters to 100 µm coated glass microspheres. It is effective in both open ambient and closed partial-pressure environments. The technique can operate at powers up to several tens of Watts.  Because the traps are distributed as lines rather than points, with optical power spread over large areas, practical implementation is most effective at powers above 1 W.
The trapping mechanism relies on positive photophoresis, which arises from asymmetric heating of the particle by the laser beam. When an absorbing metallic particle is illuminated, the hotter side emits gas molecules with greater momentum than the cooler side, resulting in a net force that pushes the particle away from the light. This photophoretic force can be expressed as:
<img alt="Photophoretic force equation F_phot = C·∇T, optical manipulation study, formula image." src="/files/ftp_upload/68766/68766eq1.jpg" style="margin:0;" data-altupdatedat="1785816036000" data-alt="Equation 1">
where C  is a coefficient that depends on the particle geometry, gas properties, and thermal accommodation coefficient, and ∇T is the temperature gradient along the particle surface. In this system, this force is directed upward in the trap and counteracts gravity, allowing stable levitation. A more detailed derivation of the photophoretic force—including both radiometric pressure and thermal creep effects—is provided in Mirzaei-Ghormish et al.14.
The boat trap uses an arbitrary waveform generator (AWG) to control acousto-optic modulators (AOMs) to form a high-intensity optical wall created by scanning a focused continuous wave (CW) laser beam to generate a stable parabolic trapping potential14 (see Figure 1B).  This approach enables precise control of the trap shape, allowing particles to be trapped, held still, and exposed on one surface—key requirements for applications such as bringing nanodiamonds close to gold plasmonic particles of dimension greater than 1 μm for Purcell enhancement of nanodiamond fluorescence11

<table><tbody><tr><td>1x, 40 mm WD CompactTL Telecentric Lens</td><td>Edmund Optics</td><td>63-745</td><td>Used to image trapped particles; telecentric lens; labeled as &#039;i&#039; in Figure 3</td></tr><tr><td>50.8 mm (2 in) N-BK7 A Coated Plano Convex Lens</td><td>Thorlabs</td><td>LA1050-A</td><td>Used for beam collimation and superposition (alignment discussed); Relay Optics; labeled as &#039;b&#039; in Figure 3</td></tr><tr><td>5x Long Working Distance LM Plan Achromatic Metallurgical Microscope Objective Lens Working Distance 15.5mm MT05073231</td><td>BoliOptics</td><td>MT05073231</td><td>fourier transforms output of AOMs; objective; labeled as &#039;g&#039; in Figure 3</td></tr><tr><td>AmScope Powerful 6 Watt LED Dual Gooseneck Illuminator</td><td>Amscope</td><td>LED-6W</td><td>Used to improve imaging contrast; White Light Source; labeled as &#039;f&#039; in Figure 3</td></tr><tr><td>AO deflector, part number 97-02799-01</td><td>Gooch &amp;amp; Housego</td><td>4120-2</td><td>Acousto-optic Modulators (AOM), y-direction scan; Y-AOM; labeled as &#039;a&#039; in Figure 3</td></tr><tr><td>AO deflector, part number 97-02799-01</td><td>Gooch &amp;amp; Housego</td><td>4120-2</td><td>AOM used for beam scanning in the x-direction; X-AOM; labeled as &#039;c&#039; in Figure 3</td></tr><tr><td>Basler ace acA640-750uc USB3, Color</td><td>Edmund Optics</td><td>33975</td><td>Used to detect particles; camera; labeled as &#039;j&#039; in Figure 3</td></tr><tr><td>Computer Running Windows 11</td><td>Dell</td><td>n/a</td><td>Equipped with data acquisition and imaging software</td></tr><tr><td>CW laser (532 nm, up to 10 W)</td><td>Coherent</td><td>Verdi V10</td><td>Illumination Laser; Laser source</td></tr><tr><td>Disposable Culture Tubes</td><td>Fisher</td><td>14-961-26</td><td>rotating chamber for particles; particle chamber; labeled as &#039;h&#039; in Figure 3</td></tr><tr><td>Glass Chamber 20 mm length 12 mm diameter Borosilicate glass culture tube cut</td><td>Fisherbrand</td><td>14-961-26</td><td>20L &amp;times; 12D mm, used for airflow stabilization</td></tr><tr><td>Glass Microspheres 40-60 &amp;micro;m ave diameter, 3M K1 &#039;bubbles&#039;</td><td>Kremer</td><td>#59910</td><td>Used for comparative testing</td></tr><tr><td>Gold Particles (1.16 &amp;micro;m mean diameter)</td><td>ASI</td><td>162-0010</td><td>Solid gold particles</td></tr><tr><td>model number TB-17</td><td>Minicircuits</td><td>15542</td><td>Used to amplify signal to AOMs; RF Amplfiers; labeled as &#039;m&#039; in Figure 3</td></tr><tr><td>Mounted Standard Iris, &amp;Oslash;25.0 mm Max Aperture, TR3 Post</td><td>Thorlabs</td><td>ID25</td><td>Used to spatial filter the diffracted orders; Aperture; labeled as &#039;d&#039; in Figure 3</td></tr><tr><td>&amp;Oslash;1&quot; 50:50 UVFS Plate Beamsplitter, Coating: 350 - 1100 nm, t = 5.0 mm</td><td>Thorlabs</td><td>BSW26</td><td>used to introduce white light along the optical axis to backlight particles; beamsplitter; labeled as &#039;e&#039; in Figure 3</td></tr><tr><td>Rigol DG4202, 2 Channel waveform generator with 200 MHz sine wave and 60 MHz square wave capabilities, 500 MSa/sec sample rate, 16 kPt arbitrary memory length, 7 inch display, and USB and ethernet (LXI) interfaces.</td><td>TestEquity</td><td>DG4202</td><td>Rigol DG4202, used for AOM control; Arbitrary waveform generator; labeled as &#039;k&#039; in Figure 3</td></tr><tr><td>ZOS-150+&amp;nbsp;</td><td>Minicircuits</td><td>M110171</td><td>Used for AOM frequency tuning; Voltage-controlled oscillators (VCOs); labeled as &#039;l&#039; in Figure 3</td></tr></tbody></table>

optically trapping large metallic particles in air using a &#39;boat&#39; trap with direct-drawn sidewalls

Optically Trapping Large Metallic Particles in Air Using a &#39;Boat&#39; Trap with Direct-Drawn Sidewalls

We present a method for achieving stable optical trapping of large metallic particles (&#62;1.16 &#181;m mean diameter solid gold particles, and &#62;100 &#956;m conductor-coated microspheres) using a focused beam, scanned by acousto-optic modulators to directly draw a parabolic trap cross-section in the air.&#160; The result of this drawing action is the formation of a &#39;boat&#39; trap, open on top, capable of catching, loading, and stably holding large metallic particles without hopping.&#160; A high-power continuous-wave 532 nm laser is scanned in both the x- and y-directions, forming a boat-shaped intensity distribution optimized for trapping microspheres. The trapping duration is maximized by careful selection of drawing frequency to be fast enough to appear continuous relative to particle dynamics, but slow enough to avoid trap distortion. This protocol describes the experimental setup, including laser modulation, particle introduction techniques, and validation. Carbon-coated microspheres are used primarily to visualize and validate trap formation and stability, while solid gold nanoparticles serve as the target for plasmonic interaction studies. This method allows for any one of a number of cross-sections to be drawn on command, in real-time, to provide a robust, flexible approach for capturing, loading, and translating airborne particles over long timescales, with potential applications in plasmonic enhancement of levitated optical systems.&#160;

Results for carbon-coated microspheres
Carbon-coated microspheres provide the most visually convenient particles to trap of the two particle types treated in this paper (i.e., carbon-coated microspheres and solid gold particles). Carbon-coated microspheres produce dark shadows when backlit, as shown in Figure 9E,F.  This makes them ideal for validating the setup of the boat trap hardware. Their high visibility makes it possible to...

This paper describes the experimental method for achieving long-duration, stable trapping and precise positioning of large metallic particles in air using a dynamic, boat-shaped photophoretic trap. This system enables controlled manipulation that is suitable for plasmonic interaction studies.

We present a method for achieving stable optical trapping of large metallic particles (&#62;1.16 &#181;m mean diameter solid gold particles, and &#62;100 ...

Our work describes the construction and operation of a reconfigurable boat-shaped optical trap for capturing and manipulating particles in air. Existing trapping methods compromise between trap localization and capture area. This protocol introduces a boat trap, achieving both simultaneously.This protocol can be applied in standard laboratory environments under ambient air conditions while following Class 4 laser safety protocols. After preparing the carbon-coated glass microspheres, set up the optical system using a kinematic beam elevator and a ruler to set the beam height to 23 centimeters. Reduce the laser power to less than 50 milliwatts.Use a polarizer to determine whether the polarization is vertically oriented. Adjust the microscope to beam height facing in the negative Z direction at a distance greater than 100 centimeters from the output of the beam elevator. Place a Fourier transform lens 10 centimeters in the negative Z direction, with the convex surface facing in the positive Z direction.Then place the remaining three Fourier transform lenses 20 centimeters apart with alternating convexities. Position the acousto-optic modulators 10 centimeters in the negative Z direction from the lenses closest to and third closest to the laser. Ensure that the acousto-optic modulator closest to the laser is vertically oriented.Position the acousto-optic modulator two 10 centimeters from the lens third closest to the laser. Align it horizontally and rotate it approximately two degrees away from the optical axis. For each channel of the arbitrary waveform generator, connect a voltage controlled oscillator, an amplifier, and one acousto-optic modulator.Then connect the amplifiers to the acousto-optic modulators. Using the arbitrary waveform generator, set the voltage supplied to both voltage controlled oscillators to five volts, corresponding to a frequency of 100 megahertz. Then decrease the tilt of acousto-optic modulator two to zero.Gradually increase the angle until the first order beam reaches its maximum brightness. Align both beams exiting the first acousto-optic modulator to enter the second modulator. Of the four beams exiting the second acousto-optic modulator, allow only the top right beam to pass through the aperture.Next, place a beam splitter to direct a portion of the beam to an observation surface. With the arbitrary waveform generator amplitudes set to zero, observe the resulting beam at the observation surface appearing as a dot. Set the vertical channel offset to five volts, amplitude to three volts, and frequency to four kilohertz.Observe the beam forming a vertical line at the observation surface. Then adjust the horizontal channel offset to five volts, the amplitude to three volts, and the frequency to four kilohertz. The beam should form a diagonal line at the observation surface.Tune the vertical channel to eight kilohertz and a 90 degree phase offset, resulting in a Lissajous pattern on the observation surface. Then press the Align Phase button to convert the Lissajous pattern to a parabolic shape. Adjust the vertical channel phase offset to refine the parabolic shape and observe the refined parabolic pattern at the observation surface.Place a telecentric lens with a 45 millimeter focal length on a linear stage, positioning it 55 millimeters in front of the objective in the positive Z direction. Then place a dichroic filter designed to reflect 532 nanometer light in front of the lens. Connect the camera to a personal computer using a Universal Serial Bus cable with the associated imaging software.Then tilt the dichroic filter slightly between five and 10 degrees until the parabolic trap shape appears. Illuminate the beam splitter with a fiber light source and adjust the fiber light to center its illumination around the parabolic trap. Using a laboratory spatula, place approximately one gram of particles inside a 25 millimeter long, 12 millimeter diameter glass cylinder mounted to allow rotation along the cylinder axis.Place the loaded glass cylinder centered at 15 millimeters away from the objective, corresponding to the objective working distance. After rotating and tapping the cylinder, observe the camera display showing default or no trapped particles, particles falling, and trapped particles. After installing and configuring the software, launch Visual Studio 2022 and open the project file.Press the green Run button to start the program. Observe the video feed and terminal appearing on the screen. Ensure that the green trapping parabola is visible on the video feed.If it is not visible, adjust power, filtering, or alignment until the trapping region appears. Press the Windows, Shift and S key simultaneously to capture the screen. Click and drag the mouse pointer from the top left to the bottom right corner of the video feed.Click the pop-up to open the screenshot editor. Click the Edit in MS Paint button within the editor. Then place the cursor in the top left corner of the parabola in the MS Paint window.Record the coordinates shown at the bottom left corner as X minimum and Y minimum. Then place the cursor in the bottom right corner of the parabola in the MS Paint window. Record the coordinates shown at the bottom left corner as X maximum and Y maximum.Once done, close the video feed by pressing the Escape key. Enter X minimum, X maximum, Y minimum, and Y maximum on line 280 in the is_trapped function of main. cpp within the project.For test frequency randomization, launch Visual Studio 2022 and open the project file if it is not already open. Open Windows PowerShell or Command Prompt. Run the command pythonc import random and print a random sample from the range zero to 101.Then copy the generated list of test numbers. Edit line 443 of main. cpp by pasting the generated list into the test number sequence and save the project in Visual Studio 2022.Increase laser power to the level needed for the experiment. Run the experiment by clicking the green run button and tapping and rotating the tube of particles. After the experiment has finished, open the results.txt file in a text editor. Observe that each program run generates new results under the heading, Start of a new batch of results, in the format, test number, result, followed by trap time in nanoseconds. For data visualization, copy the test numbers and trap times from the Comma-Separated Values file into example.xlsx.After applying the required filters, observe the additional columns and a scatter plot that populates with trap time versus trap draw frequency. Carbon-coated microspheres produce dark shadows when backlit. Glass microspheres, particles with dimensions on the order of 100 micrometers, settled approximately to the bottom of the parabolic trap.When a particle was successfully trapped, the automated particle detection software displayed a red T.When no trapped particle was found, the software displayed a green N.Trapping rates and lifetimes may increase or decrease as a function of environmental variables such as temperature or humidity. Solid gold particles on the order of one micrometer were not observable along the optical axis, but their light scatter was observed orthogonal to the axis. Video recordings confirmed that particles maintained their position for extended durations exceeding several minutes.This protocol allows a researcher to draw and quickly modify a boat trap to trap and move a wide variety of particles in the air. Future studies can integrate Field Programmable Gate Array control to enable programmable manipulation of trap shape, weighting, and cross-sectional location within a 2D plane.

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Learn to build a reconfigurable boat-shaped optical trap for capturing and manipulating large metallic particles in air using scanned laser beams. This method enables stable, long-duration trapping for plasmonic studies.

Optically Trapping Large Metallic Particles in Air Using a 'Boat' Trap with Direct-Drawn Sidewalls

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